Mobilization of nitrogen in the olive bearing shoots after foliar application of urea

Mature ‘on’ and ‘off’ ‘Manzanillo’ olives trees with three healthy branches of 10–12 cm in diameter were selected to determine changes in nitrogen levels in the bearing shoots after foliar application of urea. Each selected branch of each tree received one of the following treatments: (i) control without urea application; (ii) foliar application of urea to all the current-season leaves; and (iii) foliar application of urea to all the one-year-old leaves. Urea was applied in May, four days after full bloom in the ‘on’ year trees. Each treated leaf was immersed in a test tube containing a 4% urea solution and 0.1% Tween 20 for 5 s. Bearing shoots, composed of both one-year stems and leaves and current-season stems and leaves, were collected at intervals from the beginning of the experiment until 64 days after urea application. Nitrogen was determined in stems and leaves from each part of the bearing shoot, and in fruits during the ‘on’ year. Nitrogen uptake from the leaves was rapidly mobilized from the older to the current-season leaves of the bearing shoot, and thereafter to other storage organs of the tree or to the fruit, which is the largest nitrogen sink in the bearing shoot. No translocation of nitrogen from the current-season leaves to older leaves was observed. The rapid translocation of nitrogen from the younger leaves to other storage organs of the tree could explain the insensitivity of leaf analysis to detect excess nitrogen, since mature leaves from current-season shoots must be sampled to determine the nutritional status of the tree. The failure of leaf analysis to detect excess nitrogen may be a cause of nitrogen over-fertilization in olive orchards.